The use of glazes to provide decorative designs and finishes to the surfaces of glass, glass-ceramic, and ceramic articles had its genesis in antiquity for ceramic articles and they have been used for many years in decorating glasses and glass-ceramics. Glazes are transparent glasses which are conventionally applied to a surface in the form of finely-divided particles, customarily termed "frit", which particles are subsequently fired to a temperature sufficiently high to fuse to and develop a well-bonded, continuous glassy coating on the surface.
Commercially marketed glazes have normally contained relatively large concentrations of lead oxide (PbO) and, less frequently, rather substantial levels of cadmium oxide (CdO), each of those oxides demonstrating two properties rendering them particularly desirable as components in glazing frits. First, they act as fluxes; i.e., they reduce the melting point of the glass so it is capable of flowing along the surface of an article at a temperature which is low enough to avoid thermal deformation of the article. Second, they raise the refractive index of the glass thereby enhancing the gloss exhibited by the glaze. In addition, CdO has been utilized as a colorant in certain frit compositions. Both PbO and CdO are highly toxic, however, such that very stringent regulations have been promulgated by the Food and Drug Administration with respect to their release when compositions containing those metals come into contact with comestibles.
It has been apparent that only through the total removal of those metals from the glaze compositions would the problem of their toxicity be finally solved. Therefore, extensive research has been directed to the formulation of lead-free and cadmium-free frits which would be operable as glazes for glass, glass-ceramic, and ceramic surfaces. It can be appreciated, however, that the absence of PbO and/or CdO from the frit compositions has mandated their replacement with other constituents to confer the desired physical and chemical properties to the glazes. Thus, the frit must exhibit the following characteristics, besides manifesting an aesthetically appealing appearance:
(a) the frit must demonstrate good glass stability; i.e., it will not devitrify during the firing to coat the surface of a body;
(b) the frit must display excellent resistance to attack by acids and bases so as to avoid corrosion of the glaze coating which can result in loss of gloss, the generation of haze and/or iridescence, the development of porosity, or other defects deleterious to the appearance and/or physical character of the glaze;
(c) the fusing or maturing temperature of the frit, viz., the temperature at which the frit will exhibit sufficient flow to produce a smooth homogeneous coating, must be low enough such that thermal deformation of the article being coated is avoided;
(d) the linear coefficient of thermal expansion of the frit must be compatible with that of the surface being coated to avoid crazing and/or spalling, with the preferred frits exhibiting a linear coefficient of thermal expansion slightly lower than that of the article being coated so as to place the matured glaze in compression when the coated article is cooled to room temperature; and, if a glaze displaying a high gloss is desired,
(e) the refractive index of the frit ought to be significantly higher than that of the surface being coated; and, if the glaze-coated ware is to be used in food and beverage service and storage applications, e.g., for use as culinary ware and/or dinnerware,
(f) the frit must demonstrate high resistance to attack by acids and bases present in foods and beverages and by detergents such as are found in commercial dishwashers.
Formulations of frit compositions containing no lead and cadmium have been marketed commercially. Those frits, however, have conventionally been plagued by a particular problem; viz., each has displayed a grayish tint in the "water white" or colorless state when fired in accordance with schedules normally employed. Thus, when the glazes are observed in the transparent, colorless state, i.e., no color pigment has been intentionally added thereto, they display an undeniable grayish discoloration. As can be readily appreciated, a grayish hue is undesirable when a "water white" glaze is sought, and the presence of the gray discoloration deleteriously affects the purity of color pigmented glazes.
The customary procedure for glazing the surfaces of articles contemplates mixing frit of a desired composition (which may contain a color pigment) with an organic liquid, such as an oil or other vehicle which may also contain an organic binder, surfactant, extender, etc. That suspension of frit and medium is applied to the surface of the article and subsequently fired at a sufficiently high temperature and for a sufficient period of time to cause the frit to fuse and flow and thereby produce a defect-free coating on the article.
Scanning auger spectroscopy examination of the gray glazes indicated the presence of clumps or pockets of carbonaceous residue interspersed within the glassy phase. Stated in another way, the grayish hue is the result of incomplete removal of a carbonaceous residue produced during the maturing of the glaze. The mechanism underlying the problem has been explained thusly: because the frit particles sinter (fuse) into a solid glaze before the organic medium is completely eliminated during the firing step, a carbonaceous residue is trapped within the glassy phase which confers the grayish tint to the glaze.
Three general approaches toward solving the graying problem have been investigated: (a) milling (comminuting) the frit particles to gain an optimum mix of particle sizes; (b) selecting special organic media; and most importantly (c) modifying the parameters of the firing process. The last approach has customarily comprehended slowing the firing schedule and/or utilizing special gaseous atmospheres during the firing operation. It is readily appreciated that each of those approaches adds additional cost to the process and, consequently, exerts a negative economic impact on commercial production.
U.S. application Ser. No. 07/724,125, filed Jul. 1, 1991 by J. M. Nigrin et al. under the title ADDITIVES FOR LEAD- AND CADMIUM-FREE GLAZES, and U.S. application Ser. No. 07/724,126, filed Jul. 1, 1991 by J. M. Nigrin et al. under the title TRANSPARENT LEAD- AND CADMIUM-FREE GLAZES, provide two different solutions to the graying problem.
The former application discloses a method wherein the frit particles are coated with a strongly oxidizing metal species prior to being blended into the organic vehicle. When the frit is fired, the adsorbed metal species oxidizes the carbonaceous residue, thereby eliminating it. As expressed in terms of metal oxide, the preferred oxidizing metal species are selected from the group of Mn.sub.2 O.sub.3, SnO.sub.2, TiO.sub.2, and ZnO.sub.1-x.
The method described in the latter application involved including a substantial amount of tin oxide in the frit composition itself to act as an oxidizing agent as the frit is fired into a mature glaze.
As can be recognized from the above brief descriptions, each method required the addition of a substance not present normally in conventional frit and which remains in the matured glaze. Such additions, although relatively small in amount, exert a material effect upon the properties of the frit and the matured glaze which require compensating changes in the other components of the frits.
Therefore, the principal objective of the present invention was to find a means for treating lead-free and cadmium-free frits such that they could be rapidly fired to mature glazes without the development of a grayish discoloration, which means would not involve adding a substance to the frits which is not customarily present in conventional frit compositions and which remains in the matured glaze.
A second, but quite obviously an extremely important objective, was to accomplish the above principal objective while, at the same time, assuring that the resultant glazes retain the several critical chemical and physical properties delineated above.